The specific phosphorylated proteins found in neuronal membranes have been implicated as physiological effectors for the diverse actions of the second messenger candidates, cyclic AMP and calcium. It has been shown that calcium-dependent phosphorylation of neuronal membranes require the heat stable calcium-binding protein, calmodulin. Since the calmodulin-regulated protein phosphorylation of synaptic vesicle proteins has been linked to release of neurotransmitter from isolated vesicles, this phosphorylation system may play an important role in synaptic function. Other factors may be present in the cytosol compartment that are important in determining the steady or dynamic phosphorylation states of specific synaptic membrane proteins. We present evidence for the existence of a heat-stable factor in synaptosomal cytosol that stimulates phosphorylation of a specific synaptic membrane protein in a calcium- and calmodulin-independent fashion.